The present invention relates to a distance-velocity predicting system which is suitably used for an automobile collision predicting sensor, for example. More particularly, the present invention relates to a distance-velocity predicting system adapted to detect a danger of collision and a relative velocity immediately before a collision occurs and to generate a starting signal for life protecting equipment.
As an automobile collision predicting sensor, a back sensor has heretofore been known, which is adapted for preventing a rear-end collision at the time of traffic congestion and for preventing a collision when the car is backed up. In general, whether or not a collision is dangerous to the driver or other occupant's life depends on the relative velocity with respect to an obstacle concerned, the weight of the obstacle, the way in which the collision occurs, etc., and in many cases it is difficult to predict these things before the occurrence of a collision. As a conventional method of measuring the intensity of impact of a collision after it has occurred, a mechanical deceleration sensor is known. With the mechanical deceleration sensor, when the measured deceleration exceeds a certain threshold value, life protecting equipment, e.g., an air bag, a pretensioner, etc., is activated to relieve the impact of the collision.
Hitherto, a great variety of methods of predicting a danger of collision have been proposed. Most of the conventional predicting methods are based on the assumption that the distance between two cars concerned is relatively long, as is seen in the rear-end collision preventing system. However, it plays an important part in determining whether to activate the life protecting equipment to detect a danger of collision between a moving object and an obstacle and a relative velocity between them in a state where the moving object and the obstacle are in close proximity to each other before they come into collision.
As a conventional distance-velocity measuring method which also takes into consideration a state where the moving object and the obstacle are in close proximity to each other, a method based on triangulation is known. One example of the distance measuring method by triangulation is shown in FIG. 1. The line segment AB represents a base line length. Distance sensors are provided at the two ends A and B, respectively, of the line segment. The point P represents an obstacle. With coordinate axes taken as illustrated in the figure, the coordinates of the point P are assumed to be (X,Y). The two-dimensional position (X,Y) of the point P canbe obtained, for example, by measuring the angles .crclbar. and .phi. at the two ends of the base line AB, which is used as a reference, or by measuring the lengths of two sides BP and AP.
Distance sensors used for triangulation may be divided into two types: active and passive. The active type is adapted, for example, to emit a light beam from either or each of the points A and B to illuminate the obstacle P. Known techniques of the active type include: one in which a difference in the time required for light to reach and return from the obstacle is measured by using pulsed light; another in which the time difference is converted into a phase difference between amplitude-modulated waves to thereby measure the position of the point P; and another in which the position of a point image of the obstacle P illuminated is measured with an imaging device, e.g., a CCD, a PSD, etc.
The present applicant has already proposed one example of the passive type sensor as Japanese Patent Application No. 03-77746 (1991). The proposed technique will be roughly explained below with reference to FIGS. 2 and 3. Referring to FIG. 2, passive sensor units O.sub.1 --C.sub.1, O.sub.2 --C.sub.2, O.sub.3 --C.sub.3, and O.sub.4 --C.sub.4 show an arrangement for distance measurement. A pair of passive sensor units O.sub.1 --C.sub.1 and O.sub.2 --C.sub.2 are spaced at a relatively short distance L.sub.1, as are another pair of passive sensor units O.sub.3 --C.sub.3 and O.sub.4 --C.sub.4. These two pairs of passive sensor units are spaced at a relatively long distance L.sub.2 from one another. Let us assume that a point P on an obstacle is at a distance R from the sensor surface 1 and at a distance S from the sensor center line 3. Fixed focus lenses O.sub.1, O.sub.2, O.sub.3 and O.sub.4 are disposed on the sensor surface 1, and imaging devices (not shown) are disposed at respective positions on the imaging surface 2 near the focal points of the fixed focus lenses associated therewith. Images of the point P that are formed by the fixed focus lenses O.sub.1, O.sub.2, O.sub.3 and O.sub.4 are assumed to be C.sub.1, C.sub.2, C.sub.3 and C.sub.4, respectively- C.sub.i Q.sub.i (i=1 to 4) represents a transverse displacement of the image measured with reference to an image position C.sub.i (i=1 to 4) when the point P is at infinity. Assuming that the transverse displacement of the image is x.sub.i (i=1 to 4), the two-dimensional distances R and S may be expressed as follows: EQU R=fL.sub.2 /f{(x.sub.1 +x.sub.2)/2-(x.sub.3 +x.sub.4)/2} (1) EQU S=-R.multidot.(x.sub.1 +x.sub.2 +x.sub.3 +x.sub.4)/(4f)
where f is the focal length of each fixed focus lens, and L.sub.2 is the longer base line length. PA1 K=(V.sub.2 -V.sub.1)/(V.sub.1 +V.sub.1)
The transverse displacement x.sub.i (i=1 to 4) is determined by executing feature extraction from a correlation between the imaging devices spaced at the shorter base line length L.sub.1. It is possible according to the equation (1) to carry out distance measurement of high accuracy based on the longer base line length L.sub.2 in a short time by using correlation processing based on the shorter base line length L.sub.1.
Regarding the danger of collision, the measuring range is divided into a plurality of regions according to the degree of danger of collision on the basis of the sign of the transverse displacement and the measured distance and displayed as shown in FIG. 3. In FIG. 3, since the shorter base line length L.sub.1 is sufficiently shorter than the longer base line length L.sub.2, it is ignored. Therefore, O.sub.1 and O.sub.2 are regarded as the same point and displayed accordingly, as are O.sub.3 and O.sub.4. In addition, reference numeral 4 denotes a car, and 5 a sensor surface. Reference numerals 6 and 7 denote boundary lines. The region I is the most dangerous region. The regions II and III are lower in the degree of danger than the region I. Symbols A, B and C denote ranks of the degree of danger according to the distance. Symbol A represents the most dangerous region. In addition, R.sub.a denotes a range which is covered by active sensors annexed to the passive sensor units. R.sub.r denotes a monitoring range.
To obtain a velocity on the basis of distance measurement by triangulation, it is computed indirectly from time-series data on distance. As one example of direct measurement of a danger of collision by triangulation, the arrangement of Japanese Patent Application Post-Exam Publication No. 47-22532 (1972) is shown in FIG. 4. In the figure, reference numeral 4 denotes a moving object, e.g., a car, which is provided with Doppler sensors A.sub.L and A.sub.R. Reference symbol P denotes an obstacle including another car, V a relative velocity between the obstacle P and the moving object 4, and .crclbar..sub.L and .crclbar..sub.R are angles made between the velocity vector V on the one hand and PA.sub.L and PA.sub.R on the other. The Doppler sensors A.sub.L and A.sub.R are adapted to emit a microwave and subject the reflected wave from the obstacle to heterodyne detection to thereby detect a Doppler shift. Doppler shifts corresponding to V cos.crclbar..sub.L and V cos.crclbar..sub.R are detected by the Doppler sensors A.sub.L and A.sub.R , respectively. It is stated in Japanese Patent Application Post-Exam Publication No. 47-22532 (1972) that when the ratio between the two Doppler shifts (cos.crclbar..sub.L /cos.crclbar..sub.R) is within a predetermined range, the degree of danger of the moving object 4 and the obstacle P colliding with each other is high.
In addition, the present applicant has already proposed in the collision predicting system disclosed in Japanese Patent Application No. 04-8531 (1992) a method of obtaining an evaluation quantity concerning the danger of collision and a velocity immediately before a collision occurs. The principle of this method is illustrated in FIG. 5. Let us assume that P is an obstacle, and Q and R are distance-velocity measuring devices which are provided at both ends, respectively, of a moving object. V is a relative velocity between the moving object and the obstacle. V.sub.1 and V.sub.2 are velocity components in the directions of the distance-velocity measuring devices Q and R. .crclbar. is an angle made at the point P between the line segments PQ and PR. .crclbar..sub.1 is the angle made between V and V.sub.1. L is the length of the base line QR. The distances from Q and R to the point P are assumed to be L.sub.1 and L.sub.2, respectively. The degree of danger of collision is high when .crclbar..sub.1 .ltoreq..crclbar., and it lowers according to the extent of .crclbar..sub.1 &gt;.crclbar., which may be expressed as follow: EQU .vertline.k.vertline..ltoreq.tan.sup.2 (.crclbar./2) (2)
where .crclbar.=cos.sup.-1 (L.sub.1.sup.2 +L.sub.2.sup.2 -L.sup.2)/2L.sub.1 L.sub.2
True relative velocity V is given by EQU V=(V.sub.1.sup.2 +V.sub.2.sup.2 -2V.sub.1 V.sub.2 cos.crclbar.).sup.1/2 /sin.crclbar. (3)
When the relative velocity V exceeds a certain set value V.sub.th, life protecting equipment is activated.
The prior art suffers, however, from the following disadvantages:
In the case of active distance measurement by triangulation, the detection of a velocity with respect to an obstacle having magnitude involves difficulties stated below:
Referring to FIG. 6, let us assume that two points A and B are apart from each other by a distance corresponding to the base line length, and a distance sensor is disposed at the point A, while a pair of a light-sending unit and a distance sensor are disposed at the point B, and that a light beam BP.sub.1 is emitted from the light-sending unit, and it intersects an obstacle 8 at a point P.sub.1 at time T.sub.1. The two-dimensional position of the point P.sub.1 at this time is detected by the photosensors for position detection disposed at the points A and B. At time T.sub.2, the obstacle has moved to a position shown by reference numeral 9. The displacement vector is represented by the arrow P.sub.1 P.sub.2, where P.sub.2 represents the position of P.sub.1 at time T.sub.2. However, the point that is detected by the light beam BP.sub.1 moves on the obstacle to P.sub.2 ', not P.sub.2. Therefore, the apparent displacement vector is such as that shown by the arrow P.sub.1 P.sub.2 '. Accordingly, the displacement vector observed is different from the actual displacement vector P.sub.1 P.sub.2 in both magnitude and direction. In other words, an obstacle having of significant size involves a possibility that an erroneous displacement vector may be detected because the point illuminated by the light beam moves on the obstacle. Therefore, there is a likelihood that an accurate velocity will not be detected.
In Japanese Patent Application No. 03-77746 (1991) that uses active triangulation in combination with passive triangulation, a feature point on an obstacle is obtained by passive correlation processing, and a transverse displacement corresponding to the parallax is detected for the feature point. Therefore, there is no likelihood that the above-described erroneous detection will occur. However, in the present state of the art, the proposed method takes time and is therefore inconvenient in a case where a judgment must be made in a short time just before a collision occurs. Further, in the active measurement, no consideration is given to the detection of a velocity with respect to an obstacle having significant size.
In Japanese Patent Application Post-Exam Publication No. 47-22532 (1972), the ratio between two Doppler shifts (cos.crclbar..sub.L /cos.crclbar..sub.R) is a function of the distance between the moving object and the obstacle. Accordingly, different set values must be used for different distances, which is inconvenient. In Japanese Patent Application No. 04-8531 (1992), two different kinds of measuring devices are needed for the measurement of distance and velocity, and hence the arrangement is complicated and costly.